Add support for response context manager interface

[florimondmanca]

Prompted by #1355 (comment)

This PR is now a plain refactor that adds support for HTTPCore returning context-managed responses, should we choose to move forward with encode/httpcore#206.

This ends up basically being a re-issuing of #1341.

I know we discussed "let's just switch to context managers throughout the stack" (#1341 (comment)), but eventually all the changes in #1355 didn't seem to be worth the hassle to me. If all we want is to make the Transport API more resource-leak-safe, then it shouldn't change anything to HTTPX's API since we already handle resource closure properly here.

We could look at reorganizing things to use the context-managed interface more thoroughly internally (to simplify a bunch of try/except/finally code around close() and aclose()), but all I'm saying is that it doesn't matter as far as compatibility with encode/httpcore#206 is concerned.

[tomchristie]

Okay, after much hair-tearing I think it's time for us to call a close on this.

There's no other existing Python networking libraries that adopts this approach wrt. handling the network close, and while it does have some point to recommend it, it also results in longer tracebacks, more awkward under the hood wrangling, and ultimately a Transport API that'd be less obvious to most of our user base, than a standard ol' file-like-ish stream with .close()/.aclose() methods.

[adriangb]

There's no other existing Python networking libraries that adopts this approach wrt. handling the network close

@tomchristie I may be missing something, but doesn't aiohttp exclusively use context managers?

[tomchristie]

doesn't aiohttp exclusively use context managers?

I'm not sure what the API for that was at the time I originally made that note.

So...

Yes they do. At the response level.

But not within the lower-level stack or at their Connector API layer.

We also exclusively use context managers at the response level. Ie. You need to use a client.stream block if you want to hold the connection open, while other methods return a closed response.

But we don't use them at the transport API.